FIX: Allow as_grey on RGBA images.

Fix bug in PIMS that broke `as_grey` for images with an alpha channel.
Any particular reason for taking 1.0 as a calibration for the alpha channel? In `skimage.color.colorconv` they ignore alpha channel, that is equal to taking 0.0 as calibration. In either case, the sum of the calibration should be 1 otherwise it will be clipped.